This will be a multi-center, prospective, double-blind clinical trial of physiologic (5%) oxygen tension in culture media vs. standard of care, atmospheric (SOC, 20%) oxygen tension with 840 eligible couples recruited to participate. The randomization scheme will be coordinated through the central data coordinating center (DCC-Yale) and the randomization will be stratified by age group of the woman (18-34, 35-37, 38-40 and 40-42) and each participating site.